New insight into surface wetting of coal with varying coalification degree: An experimental and molecular dynamics simulation study

The surface wettability is an essential factor in coal flotation and varies with the degree of coalification. Basic understanding of the wetting mechanism of different coal types is a prerequisite for proposing a new enhanced coal flotation method. In this study, the surface wettability of coal with varying coalification degrees, namely low-rank coal (LRC), bituminous coal (BC), and anthracite coal (AC), was investigated using contact angle measurements, X-ray photoelectron spectroscopy (XPS), and molecular dynamics (MD) simulations. The contact angle tests demonstrated that the water contact angle decreased in the following order: BC, AC, and LRC. This was due to the increased oxygen-containing function groups, as confirmed by the XPS analysis; the variation trend of the contact angle was further verified via MD. In the simulations, the variation of the contact angle of the simulated calculation agreed with the experimental results, suggesting that the set of parameters chosen for the simulation were appropriate. The spread of water molecules on the coal surface was highly related to the hydrogen bonds formed between water and coal, as directly confirmed by the statistical analysis of the number of hydrogen bonds. LRC and BC had the highest and lowest number of observed hydrogen bonds, respectively.